A white light-emitting LED device, which is a single device comprising a fluorescent substance and a semi-conductor light-emitting element such as LED in combination so as to give off white light, is becoming popularly used nowadays as a light source for illumination and the like. The fluorescent substance employed in the white light-emitting LED device is required to exhibit high luminous efficiency when excited by near UV to blue light. For use as that fluorescent substance, various oxynitride fluorescent substances have been proposed. For instance, WO07/105,631 discloses a green light-emitting oxynitride fluorescent substance, whose crystal structure is disclosed to be the same as Sr3Al3Si13O2N21 in JP-A 2010-31201 (KOKAI).
In a conventional process for producing a green light-emitting SiAlON phosphor, materials such as SrCO3, AlN, Si3N4 and Eu2O3 are mixed and then fired in a nitrogen atmosphere under high pressure. However, this process has a problem of forming a considerable amount of by-products (variant phase crystals) that emit luminescence of colors other than green.
For the purpose of preventing the formation of the by-products, it is studied, for example, to adopt Sr3N2, EuN, Si3N4, Al2O3 and AlN as the materials that are mixed and then fired in a nitrogen atmosphere under high pressure. However, although this method enables to prevent the formation of the by-products, Sr3N2 used as the Sr source in the method is chemically so unstable as to decompose readily in air and hence is very difficult to handle. In addition, Sr3N2 is an expensive compound.